Magnetic sheet coatings



Patented Mar. 22, 1949 MAGNETIC SHEET COATINGS Paul L. Schmidt, Perrysville, and John A. Campbell, Pittsburgh, Pa, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a. corporation of Pennsylvania Application June 8, 194.6, Serial No. 675,433

lClaim. 1

This invention relates to compositions suitable for application to magnetic sheet material, and the products produced therefrom.

It has been proposed to apply various compositions to magnetic sheet material in order to provide insulating coatings thereon. Such insulating coatings reduce eddy current losses and thereby improve the efficiency of apparatus embodying such magnetic material. However, numerous short-comings have been found in the compositions heretofore employed in the art for coating magnetic material. Sodium silicate coatings have the disadvantage that, when exposed to dampness, they will hydrolyze and produce sodium hydroxide. The resulting alkali causes rusting and corrosion to take place. Further, paints or insulating varnishes applied to electrical apparatus in which sodium silicate has been applied to the laminations often will fail due to alkaline products being formed.

Ordinary organic finishes, such, for example, as shellac, phenolic resins and alkyd resins, have been tried, but have been found deficient with respect to a satisfactory ohmic resistance and space factor. It is necessary that the ohmic resistance of a suitable insulating coating for magnetic sheets or laminations should have a median value of at least two ohms per square centimeter for most applications. Insulating coatings of slightly higher ohmic resistance are desirable for laminations of great width. Thus, for a large stator punching having a dimension of six inches from the slot portion to the outer edge, the median resistance for an insulating film should be about ten ohms per square centimeter. Higher specific surface resistance gives a correspondingly better core performance. Furthermore, for most applications, the ohmic resistance should not be less than 0.4 ohm per square centimeter for 90% of the laminations. These requirements of ohmic resistance are extremely'difficult to meet unless excessively thick'layers ofresin are ap-'- plied.

An additional defect encountered in the used such ordinary organic finishes is the relatively great thickness thereof resulting from any common method of application, which causesan undesirable decrease in the space. factor of the magnetic material. Using sheets coated on both sides, the space factor for best structural ef iciency should be such that at least 96% of the resultant core should be composed of the magnetic material and less than 4% be composed of the insulating coating. For 13 mil thick magnetic sheets, the coating should be less than mil thick. The term space factor as employed herein means the percentage of magnetic material in a given core volume.

Other disadvantages present in ordinary organic coating materials are that they require the use of solvents composed entirely of volatile organic solvents for ease of application. Organic solvents are not only costly, but, more important, are flammable and may be explosive and often are toxic.

Many of these ordinary organic materials can not be applied effectively by roller coating but must be dipped and drained. Dipping of magnetic sheet steel is not desirable in many cases because of certain coating problems encountered, such as uneven thickness, running and the like. Roller coating methods are preferable for applying insulating coatings to magnetic sheet material.

The object of the present invention is to provide magnetic sheet material with a melamineformaldehyde resin coating having a high ohmic resistance and of relatively low thickness.

A further object of the invention is to provide a relatively non-inflammable composition carrying melamine-formaldehyde resin, the composition composed of water as the main solvent component.

Other objects of the invention will, in part, be. obvious .and will, in part, appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had tothe following detailed description and drawing, inwhich the single figure is a graph plotting resistance values.

We have discovered that thermosetting aminotriazine-aldehyde resins, particularly melamine-formaldehyde resins, possess unexpected characteristics not possessed by other organic resinous materials which enable unusually effective use for insulating films on sheet magnetic material without excessive thicknesses being Ite- 'q-uired. Thusfor example, me1amine-formaldehyde resins in, a partially reacted state may be readily dissolvetlin, a solvent composed of water as. thermal or ingredient and a minor proportion of a. Water-soluble volatile organic solvent. The melamine-formaldehyde resins may be cured to a thermoset state by brief heat-treatment at temperatures above, C. The solutions of melamine-formaldehyde resin may be applied to electrical steel with felt or rubber rolls so that extremely thin coatings that are quite continuous arev produced. In many cases, three successive coatings have been applied whose total thickness is only a fraction of a mil. These coatings were glossy and bone hard so that laminations, such as slotted motor laminations, could be readily stacked and shifted or drifted to a predetermined position with respect to one another. The coatings of the aminotriazine resins are tenaciously adherent and have been found to be impervious to moisture, oils, and commonly encountered solvents. The electrical resistance has been outstanding, particularly when the extreme thinness of the films is considered.

For the purpose of this invention, one mole of an aminotriazine having the following unit formula:

where R is a radical selected from the group consisting of amino, alkyl, halogen, substituted amine, alkoxy and hydroxy radicals, is reacted with from 1.2 to 4.5 moles of an aldehyde, such as formaldehyde, to a soluble and fusible resinous state, and capable of being reacted further to a thermoset state.

Other suitable aldehydes are paraformaldehyde, furfuraldehyde, butyraldehyde and acetaldehyde. Hereafter the invention will be described with specific reference to the reaction product of melamine, which is 1,3,5,triamino-2,4,6-triazine, and formaldehyde. The reaction may be carried out in the presence of an alkaline catalyst, such, for example, as lime, barium hydroxide, ammonia, ethylene diamine, and propylene diamine. The amount of catalyst is of the order of one-half percent. The reaction is preferably carried out so that a partially reacted melamine-formaldehyde resin is produced which is soluble in hot water at 70 C. However, a certain latitude in the degree of reaction with respect to water solubility is permissible since the reaction product may be dissolved in a mixture of water and a minor proportion of a water-soluble organic solvent therefor.

While hot water may be employed as a carrier or solvent in some cases, solvents for the partially reacted melamine-formaldehyde resin for greater latitude of use in the practice of this invention are composed of from 90 to 25 parts by weight of water and from 5 to 25 parts by weight of a Watersoluble organic solvent having a, boiling point about that of water or less, and capable of dissolving the melamine-formaldehyde resin. It is preferable to have the water equal at least twice the weight of the organic solvent. Simple watersoluble alcohols, such as ethanol, propanol, isopropanol and butanol, ethanolamines, morpholine, and acetone are suitable organic solvents for this purpose. If the boiling point of the organic solvent exceeds 120 C., on oven drying the Water will evaporate first and the final traces of residual organic solvent on the sheets may catch fire.

A coating composition comprising from 5% to 70% by weight of the partially reacted melamineformaldehyde resin, from 90% to 25% water and 5% to 25% of organic solvent has been found suitable for producing thin insulating films of adequate ohmic resistance. Compositions having from 5% to of the melamine resin produce exceedingly thin coatings. With compositions having a greater percentage of resin the coatings are of higher ohmic resistance though being thicker.

Since the magnetic sheet material may be slightly greasy and will not be readily wetted by the aqueous base solvent composition employed, it may be desirable to add from 0.01% to 3% of a wetting agent thereto. Suitable wetting agents are sulphonated alcohols, long carbon chain sulphates, such, for example, as lauryl sulphate, and the salts of fatty acids and various soaps, for example, sodium stearate.

The following examples illustrate the practice of the invention:

Example I A composition composed of 10 parts by weight of partially reacted melamine-formaldehyde resin, 10 parts by weight of ethanol and 80 parts of water were admixed into a homogeneous solution. To the solution .5% of the dioctyl ester of sodium sulfosuccinic acid and .5% of methyl cellulose were added to the composition to improve the viscosity and wettin characteristics thereof. Strips of magnetic material and punchings were coated with the solution by means of rubber rolls. The magnetic sheets were immediately passed through a gas-fired furnace where they reached a temperature of 0., being exposed to this temperature for five seconds. The laminations were coated three times. After the last coating, the laminations were passed through the furnace a second time in order to completely thermoset the resin.

The electrical resistance of the laminations coated with the resin of this example were determined for a arge number of samples and plotted as curve A in the figure of the drawing. t will be apparent that the median resistance is 4 ,4; ohms per square centimeter, and that less than 3% of the laminations have a resistance of one ohm per square centimeter. These values are well within the requirements for electrical sheet steel.

The space factor of the cores produced from laminations carrying the coating of this inven tion was better than 99.5%.

Example II A solution was prepared from 25 parts by weight of melamine-formaldehyde resin, 15 parts isopropyl alcohol and 60 parts water. One-fourth of one percent by weight of the Wetting agent was added to the composition. The composition was applied to motor core punchings by means of a felt roll, Three coatings were applied, the coated laminations being heat-treated for five seconds at a temperature of about 150 C. less after each application, with an extra heat-treatment after the third coating. The distribution of electrical resistance for a great number of laminations is shown in the figure of the drawing at curve B. The median resistance was better than 12%; ohms per square centimeter. Less than 0.01% of the laminations had a resistance value below 0.4 ohm per square centimeter. Cores produced from the laminations had a space factor of 99.05%.

Example III A composition was prepared from 35 parts of melamine-formaldehyde resin, 15 parts ispropyl alcohol and 50 parts water. Three coatings of the composition were applied to punchings of electrical sheet steel by means of felt rolls. Afterheat-treating in accordance with the procedure set forth in Example I, the punchings were tested for electrical resistance. Curve C in the drawing illustrates the distribution of resistance values. The median resistance value was 50 ohms per square centimeter. A negligible proportion of the laminations had a resistance below three ohms per square centimeter. Cores built up from the laminations had a space factor of 99%.

The melamine-formaldehyde resins may be combined with finely divided solids in order to provide for desirable improvement in properties, such as viscosity, electrical resistance, and coating thickness. The solids should not exceed 35% of the weight of the resin. Up to 15 parts by weight of finely divided solids may be suspended in the compositions for such purposes. Examples of finely divided solids for this use are carbon, iron oxide, silica flour, mica dust, alumina, magnesia, asbestos fibers and bentonite.

The following examples illustrate this feature of the invention:

Example IV 32 parts by Weight of melamine-formaldehyde resin, 14 parts of isopropyl alcohol, 8 parts of lampblack and 46 parts of water were admixed into a uniform composition, The composition was applied by felt rolls to laminations and sheets of magnetic material. When applied and heat-treated in accordance with the procedure set forth in Example I, the resulting laminations had resistances shown in curve D of the drawing. The median resistance was 8'75 ohms per square centimeter. The space factor of a core prepared from the laminations Was 98 /2%.

Example V 24 parts melamine-formaldehyde resin, 14 parts isopropyl alcohol, 57 parts of water and 5 parts of iron oxide were combined into a homogeneous composition. The composition was applied to laminations in three coatings and heat-treated as set forth in Example I. Laminations provided with the coating were found to have a median resistance of better than ohms per square centimeter.

Various types of sheet magnetic material may be treated with the compositions of this invention. Silicon iron having from 1% to 7% silicon is readily coated with the compositions. Other ferrous alloys, such, for example, as cobalt-iron alloys may be treated with the compositions disclosed herein. Powdered magnetic material, such as iron and nickel, may be coated with the resins of this invention and compacted under heat and pressure to produce resin bonded magnetic cores suitable for various applications in radio and other fields.

The coated sheet magnetic material may be punched after the resin curing heat-treatment in order to produce rotor and stator laminations. transformer punchings, and other magnetic laminations. The coating has been found to prolong die life as compared to other conventional coatings, such as sodium silicate.

If desired, the resin coated laminations of magnetic material may be annealed to remove strains set up by the punching operation. However, the annealing temperatures will decompose the resin and the laminations must be recoated in order to provide an insulating film thereon.

When embodied in cores, rotors and stators, the coated laminations produced in accordance with this invention may be subjected to varnish impregnation and other conventional insulating treatments without causing undesirable changes in the varnishes or other insulating compounds. Paints and insulating compositions will adhere to the coated laminations even under extreme conditions of temperature and humidity,

Since certain changes in the above process and the compositions and resulting products may be made without departing from the scope of the invention, it is intended that all matter set forth herein shall be considered as illustrative and not in a limiting sense.

We claim as our invention:

Insulated magnetic sheet material comprising, in combination, magnetic sheet material and an insulating film applied to a surface of the sheet material, the film composed of thermoset melamine-formaldehyd resin, the film not exceeding 2% of the thickness of the sheet material, and finely divided solids distributed in the melamineformaldehyde resin in an amount of less than 35% of the weight of the resin, the solids being selected from the group consisting of iron oxide, silica, alumina, magnesia, asbestos, mica dust and bentonite.

PAUL L. SCHMIDT. JOHN A. CAMPBELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,255,901 Schroy Sept. 16, 1941 2,270,662 Raney Jan. 20, 1942 2,291,079 Hofierbert July 28, 1942 FOREIGN PATENTS Number Country Date 552,152 Great Britain Mar. 25, 1943 

